There is a danger of creating a "parallel civil service" if the number of special advisers is increased, MPs have been warned.

MPs on the Commons public administration select committee, which is carrying out an inquiry into the role of special advisers, have been told by Prof Martin Smith of the University of Sheffield, that if a minister had 10 or 15 special advisers they would no longer need a private office and would run the risk of creating a "parallel civil service".

Giving evidence to the committee, Smith said the danger was "some way off", but said it highlighted a lack of clarity about the terms on which special advisers, and other advisers to the government, such as specialist tsars, are employed. "It is a general question about what a civil service contract means," he said.

Zoe Gruhn from the Institute for Government thinktank, told the MPs that special advisers need to be employed on a civil service contract because they are part of the government machine.

Dr Andrew Blick, of Democratic Audit, said the special adviser system is at least regulated to some degree, which he described as preferable to having special advisers "floating in some nether region".

Gruhn said raising the transparency of the way in which special advisers are employed would be a good idea, to ensure getting the best possible person for the role and the more transparent that process the better, although she felt that pre-appointment hearings, suggested by some MPs, would not be a good way to achieve this. "I think there is the need for some kind of public appointment process," she said, possibly for the most high-profile, senior appointments.